Process for roasting ore



Sept. 26, 1939. B.' M. CARTER 2,174,185

PRocEss FOR RoAsTING ORE Filed March 16, 1938 2 sheets-sheet 1 III SQSQ,

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Patented Sept. 26, 1939` PATENT oFFlcE PROCESS ron RoAsTlNG one Bernard M. Carter, Montclair, N. J., assignor to General Chemical Company, New York, N. Y., a corporation of New York application March 1s, 193s, serial No. 196,201

17 Claims.

This invention relates to roasting of metal sulfide iines by suspension methods of the type disclosed in Cordy and Burgoyne United States Patent 1,758,188 of May 13, 1930.

Oxidation of metal suliide, iron pyrites for example, is an exothermic reaction liberating large quantities of heat. Ina situation where a commercial suspension roaster of given size is operated to produce, at the burner exit, say a 10 SO2 gas, there is a maximum pyrites tonnage which may be satisfactorily desulfurized in a day. With normal radiation from the furnace shell, the heat evolved by combustion of the pyrites with production of a 10% SO2 gasis distributed, in a representative operation, approximately as-follows: the gaseous products of combustion 'I0-80%; radiation loss i0-20%; cinder 10%. In commercial operation, the gas temperature prevailing in suspension burners ranges from about 1800 F. to about 1900" F. and is generally around 1850 F. The gas vtemperatures in the burner should not be permitted torise appreciably above a maximum of say 1900 F., especially inthe zone in which cinder collects just prior to discharge from the furnace, because the iron oxide cinder would cease to be free-flowing and would lbecome soft and sticky, thus'preventing normal discharge of c inder from the furnace and otherwise interfering with operation to the extent of rendering the same impracticable. In' a given burner, the'- total amount of heat generated is directly proportional to the amount of pyrites burned, heat loss by cinder is relatively small and is directly proportional 35 to the ore charge, and heat loss by radiation is constant. Since gas temperatures should not exceed a maximum, it will be seen a given furnace has a miximum pyrites capacity. For

, example, it may be assumed that in production of a 10% SO: gas, the maximum daily pyrtes capacity of such given furnace is 50 tons.

It would be highly desirable to operate such a burner so as to produce a stronger say 12% S102 45 gas to permit utilization of a smaller wet purication and converter units of the succeeding HzSO4 plant. However, since in production of strong SO2 gas, avoidance of excessive temperatures may be had only by cutting down the 50 amount of suliide ore burned, in order to produce the stron-ger 12% SO2 gas and yet prevent burner exit gas temperatures in excess of permisslble maximum, it would be necessary to reduce the sulilde ore charge to roughly tons. 55 One of the objects of the present invention is to provide a method by which a given burner of the type disclosed in the Cordy patent may be operated so as to produce the strong SO2 gas desired without (as hereafter explained) having to reduce, for example by about 40%, the amount of sulfur fed into the burner.. The invention aims to provide a sulfide ore roasting process by which all plant units may be substantially reduced in size, or alternatively by which the H2SO4 production of all existing plant units succeeding a 104 given burner may be largely increased. Another object of the invention is to accomplish the desired resul-ts by use, in conjunction with the roasting of the metal sulfide lines, of iron sulfate which is of little if any market value and which 15 for all practical purposes-may be considered a Waste product. A further object of the invention is to provide a method for the economic recovery of the sulfur and iron Values contained in iron sulfate. 20

Certain industrial operations, for example pickling of steel and manufacture of titanium pigments, result in production of large quantities i of waste or spent liquors containing as their chief ingredients free sulfuric acid and iron sulfate 25 1 and will in no case yield a return on investment since no economic method of recovering values of iron sulfate is available, and there is no tonnage outlet for iron s fate either in the form of the monohydrate, eSO4.I-I20, or the heptahydrate FeSO4f7H2O.

Another object of the inventionis to provide' such a method for the recovery of iron and sulfur from iron sulfate that it becomes possible to 40 dispose of Waste Il'zSOi-FeSOi liquors in such a way as to not only cover disposal and operating costs but lalso to provide a return on 'the disposal plant investment.

Although the process of the invention is readily adaptable to use of several other substances as initial raw materials, speaking with respect to the preferred embodiment in which iron sulilde and iron sulfate are employed, I havexfound that by introducing iron sulfide nes, iron sulfate lines, and oxygen into a suspension roaster combustion 'zone heated to temperatures above the ignition point of the iron sulfide iines, and by regulating the amounts of iron sulildeflnes, iron sulfate nes and oxygen introduced so that the 6.6

2 routing operaiioniseectedinsuchawayas to maintain in the combustion lone temperatures about or above diociation temperature of sulfur trioxide but preferably not above temperatures i whichwouldcauseironoxidecinderatthepoint oidiacbargefromtheroastertohxacandsetup aaa solid cake, the iron sulildeiines're'roasted to produce iron oxide cinder and sulfur dioxide gas; the iron sulfate is decomposed to form iron lo oxide cinder, sulfur diode and sulfur trioxide gases; the sulfur trioxide formed by decompositionoftheironsulfateisdissociatedtosulfur dioxide and. oxygen; and the sulfur and iron valuesoftheironsuliideandof tbeironsulfate are recovered as a concentrated sulfur dioxide gas and as relatively free-flowing iron oxide cinder.

I have found that by so proceeding, with respect to a given burner,` the iron suliide input I) .limitation previously mentioned is eliminated and the iron sulfide capacity is materially increased; limitation as to SO: concentration is eliminated; the actual and potential heat in a burner, that is actual and potential heat, over and above that needed to keep the burner at operating temperatures is Vabsorbed by and used to decompo iron sulfate. Further, decomposition of iron sulfate to iron oxide and S: is effected without consumption of carbonaceous reducing agent and lo without taking any special steps, other than those usually involved in suspension roasting of sulfide' lines, to create in the combustion zone tempera- `tures high enough to dissociate S0: to SO: and oxygen. Iron sulfate is decomposed to form the usually sought for suspension roasting products, iron oxide and 80:, without incurring expense of independent operations necessitating apparatus, fuel. and operating charges. By reason of the facility with which a concentrated SO: gas may 0 be produced, hereinafter explained material reduction in size may be obtained in each plant unit, namely, the burner, the puriiication system, the drying unit, the converter system, and

the absorption system. Furthermore, I am l enabled to secure all of these and further advantages which will appear as the description' proceeds by utilizing in conjunctionwith the suspension roasting of iron suliides a material, iron sulfate, for which there is no appreciable market '50 outlet and for which reason economic recovery of iron and sulfur values from waste industrial HaSOq-FeSOi liquors has not been possible previously.

Pig. lofthedrawingsillustratespartlyinsec- VIl 'tion and partly atically a plant layout in which a preferred embodiment of the invention may be carried out: and Fig. 2 similarly represents apparatus in which a modiiied process may be practiced. .Referringto Fig. Lshaft furnace Il comprises a vertically elongated preferably cylindrical steel shell il. Since on account o f the nature of the invention it is now possible to use in the roasting operation all of the'heat which can be generated by combustion of suliides, radiation of heat from the burner shell need no longerbe relied upon to provide for dissipation of some excess heat.

Accordingly, the burner is preferably well insulated to cut down radiation heat loss as `far as 31.0 feasible, and forthisreason alayer I4 of insulat-4 "ing materialsuchasSilocelmaybeinterposed'v hetweenshellilandthecustounaryiirebricl:lin-

ing I5.

A mixture of iinely divided sulfide ore and `nnnelydividedironsulfateinairor vother oxidizing gas is fed into the upper end of roasting chamber Il by feed mechanism I8 shown diagrammatically on the drawings. The feed delvice may include an upwardly projecting cylindrlcal casing having the lower end set into 5 the crown of the furnace and the upper end closed ol! by plate 2l provided with a circular opening to accommodate a sulilde-sulfate mixture yinlet conduit 23. The diameter of conduit 23 is considerably less than that of casing 20 so as to form an annular air chamber 25 between the outer surface of the'conduit and the inner surface of casing 20. A mixture of iinely divided sulfide ore and iron sulfate may be continuously fed into conduit 23 by' a screw conveyor 28 associated with hopper 21. l-

Air, serving to effect the suspension of sulde and iron sulfate nnes and to support combustion of sulfides in the roasting chamber, is introduced into the feed mechanism through the inlet 30. The conduit 23 may be provided with a series of openings 3| to permit admission of air from the annular chamber 25 into the interior of conduit 23 to bring about suspension of sulde iines and iron sulfate fines in air. The particular feed mechanism illustrated constitutes no part of the present invention. Accordingly, any desirable means for forming a suspension of sulfide and iron sulfate i'lnes in oxidizing gas and introducing such suspension into the upper end of the roasting chamber I1 may be employed. For example, mechanism adapted for this purpose is fully described in the Cordy patent.

The bottom of the roasting chamber is funnelshaped and is provided with a suitable device 32 for removing cinder from the furnace at any desired rate. The SO: gas outlet 33 communicates through conduit 34 with the bottom of a supplemental combustion and dust settling chamber 35 provided at the bottom with a dust discharge air. lock 38: The top of chamber 35 is connected by flue 31 with inlet header chamber Il of heat exchanger 40, the inwardly sloping lower portion of the walls of which terminates in an air-lock 4l arranged to permit removal of dust without admission of air. Transferrer 40 may be of any approved construction, although in the embodiment illustrated, is the fire-tube type, an incoming air heating chamber being formed by the exterior of tubes`43, the upper and lower tube sheets, and the intermediate inner `surface of the walls of the shell. Since the SO2 gases introduced into the top of transferrer 40, are highly destructive at the prevailing high temperatures, caution `should .be taken to construct all metal parts in contact with the gases of sufflciently resistant material such as chrome steel. The gas outlet header chamber is connected through gas conduit 45 with inlet header chamber 48 of a second Aheat exchanger 5l) provided at the bottom with a dustl discharge air-lock 5I.v Design of exchanger 50 is the same as that of 40. The reason for using two exchangers is that since SO: gases entering the bottom of exchanger 50 are considerably cooler than the burner exit gases; the tube section of exchanger 50 maybe ,ordinary grade of steel. Exchangers` W n l are preferably arranged with tubes 43r,and. 5ft-vertical so that dust separating from the SO:` v 'gases may drop into the lower lends of the transferrers and not clog up the tubes. y

Air is pumped by blower 53 through conduit 54 into the space surrounding tubes 56,in exchanger 5l, through conduit 58 into the space surrounding tubes 4I in transferrer 40 and then through line 6I to feed mechanism air inlet 30.

das outlet header chamber 82 of transferrer 58 is connected through conduit with a cyclone dust separator 85 which may be comparatively small in size and made of ordinary sheet iron or steel. Gases from which major portion of the dust has been removed by separator B5 pass through line 86 to a wet puriiication plant 81 which may be ofthe type disclosed in the Herreshofl. Patents 940,595 of November 16, 1909, and 1,113,437 of Cctober 13, 1914. Drying system 98, blower 68', contact unit 69 and the absorption system 19 may be of standard design.

In the apparatus. of Fig. 2, shaft furnace 1-2, dust collector 13, wet purification plant 14,'drying system 15, contact unit 18, and absorption system l1 are the same as in Fig. 1. However, in the operation of the process as carried out in the apparatus ofFig. 2 the air consumed in combustion chamber 18 oi' furnace l2 is fed into the system by a blower 80, passed through a standard drying tower 8|, and introduced through line 82 int'o air inlet 30 of feed.mechanism I8.

Gas outlet 84 of furnace 12 communicates through conduit 85 with the bottom of a supplemental combustion and dust chamber 81 provided at the bottom with a dust discharge airlock 88. 'I'he top of chamber 81 is connected by pipe 90 with the inlet header chamber 92 of a Waste heat boiler 94, the lower portion of the Walls of which terminates in an air-lock 95 Aarranged so as to permit discharge of dust without admission of air. Boiler 94 may be of any approved construction, and inthe embodiment illustrated is of the rire-tube type, the water and steam chamber being formed by the exterior of tubes 96, the upper and lower tube sheets, and the intermediate inner surface of the shell walls. Water may be fed in the space surrounding the tubes through an inlet 98, and steam or hot water withdrawn through outlet 99'. A suitable pressure regulator isV indicated at |00. Gas outlet header chamber I02is connected through conduit |04 with a cyclone dust separator 13. s

An understanding of the nature of the invention and of its many operating advantages may be had by initially considering, as a basis for comparison, what may be deemed a representative example of prior operation of a pyrites suspension burner and the associated H2SO4 plant. Such plant comprises ve principal units: suspension burner; Wet purification system, drying unit; converter unit; and an S03 absorption system. As a starting point, it may be assuned the gas purification system is in accordance with conventional practice and initially designed to handle a gas, say of SO2 concentration. Hence, by regulation of the amounts of sulde lines and air introduced, the burner is operated so as to produce a gas containing about 10% SO2 and 8% oxygen. In this situation, the Wet purification exit gas would likewise contain about 10% SO2 and about 8% oxygen. While theoretically one volume of SO2 reacts with 0.5 'volume oi' Oz to form S03 in the converters, in practice in order to obtain commercial conversion the amount of oxygen needed should be in excess of the theoretical requirement, and for practical purposes the gas Afed to the converter should contain SO2 and O2 in ratio of about 1:1.1. .'Ihus, in the case of a 10% SO: pyrites gas, more air should be fed into the gas stream to provide the requisite oxygen in the converter. This may be accomplished by introducing the additional air needed just ahead of the drying system, as through an air inlet |38, Fig. 1 of the drawings. In the case Jtration of about 8.5%.

` of a 10% SO2 pyrites burner gas, the extra air needed dilutes the gas stream to an SO: concen- Accordingly, where the purication system is initially designed for a 10% SO2 gas, the converter unit would be designed to handle an 8.5% SO2 gas.

Example 1 loi? Table I appendedto this specilication summarizes operation of such a plant. The burner used (Item 2) is a. given standard, for instance as shown in the Cordy patent. The puriiication system (Item 11) is designed for a 10% SO2 pyrites gas, and the drying, converter and absorption systems (Item 15), for reasons just indicated, are designed to handle an 8.5% SO2 gas. For reasons explained in the second paragraph of this specification, in the produc-- tion of Va gas of given SO2 concentration a given vfurnace has a definite maximum pyrites capacity. It may be assumed that in the production of a 10% SO2 gas the maximum daily pyrites capacity of such given furnace is 50 tons (Example l, Item 3) and on this basis it may be considered that the burner is operated at 100% (sulfur output) capacity. The temperature of the burner exit gas is l850 F. (Item 9).

On account of high construction and maintenance costs of the puriiication system, it would be highly desirable to operate the burner to form a stronger gas, say 12% SO2 to permit utilization of smaller purication units. .The given burner is readily adaptable to produce a stronger gas. However, it will be understood that in formation of a 12% SO2 gas, the amount of air introduced into the furnace is necessarily reduced, and the temperature of the gas in roasting zone tends to rise correspondingly. The only way to avoid excessive temperatures'in the combustion zone is by reducing the tonnage off ore fed into the,

roaster. For example, commercial practice has shown that where a given burner had a daily pyrites capacity of about 50 tons on production of a 10% SO2 gas, when the same burner was operated with the same ore and at the same burner temperature so as to supply a. 12% SO2 gas, it was necessary to cut down the ore feed to the burner to about tons. Such an operation is summarized in Example 2 of Table I which shows that to supply the stronger gas to puriiication, the burner can be operated at only 58% capacity with the result that the H2804 outputv of the plant as a whole is only 58% of the normal output of Example 1.

If an operator wishes to feed to purification a 12% gas for the purpose of effecting economies resulting from the use of a. smaller purication unit and at the sa'me time maintain his normal H2SO4 production he has available as one alternative installation of an additional or supplementary burner about' 70% aslarge as the initial given burner. summarized in Example 3, the plant of which Operation of this alternative isrequires construction'and use of a supplemental ore capacity), an creased percentageotthe total heat generated would normally be carriedV 'burner with the SO2 gases is limited (so as to avoid softening of the cinder), in order to provide for disposal by radiation of the greater amount of heat generated it would be necessary to build a burner having about 2.5 times the combustion space of the initial given burner. This obviously involves greatly increased cost of the burner unit. Operation of the plant using a larger new burner is summarized in Eixample 4 of Table I.

The present invention not only provides elimination of the foregoing dimculties but affords many other substantial operating advantages which will become apparent as the description proceeds.

In the following specific example of practice of the invention, raw materials used are iinely divided iron pyrites, for example flotation concentrates, and iron sulfate in the form of the monohydrate FeSO4.HzO, supplies of which materials are maintained in bins I and Ill. At start of operations, combustion chamber l1 of furnace I is heated by oil. burners inserted through workholes not shown to temperatures of 900--1000 F. Pyrites fines alone are then injected and burned until the roaster temperature is raised to about normal, e. g., 1800-1850" F. Subsequently, flnely divided pyxites and FeSOi.- HsO are` fed into hopper 21 through conduits lli and |30, proper proportions being maintained by regulation of valves |31 and IIB. The raw materials are mixed in conveyor and charged into the top of feed conduit 2l. In the preferredy embodiment, the air used to supply combustion in the furnace is preheated in heat exchangers Il f and l0. Assuming furnace I 0, dust chamberA and exchangers and 50 are heated up to normal operating temperatures, air at temperature of about 100 F. is introduced by blower Il through pipe Il into the air space in exchanger 50, and heated to temperatures of about 550 F. Such ai! flows through pipe 5l and is introduced at temperature of about 500 F. into the air space of exchanger !0. The air is discharged through pipe 0l at temperature of about 950 F., and is conducted by pipe to air inlet 30 of furnace `feed mechanism Il which may be of any approved design to effect introduction into combustion chamber i1 of a suspension of finely divided iron pyrites and iron sulfate in air.

On introduction into chamber l1, heated to temperatures above the ignition point of iron sulde, the pyrites iiash to ignition and roasting commences. Suspension roasting of pyrites is highly exothermic and results in formation of a gas mixture comprising SO2, oxygen and nitrogen and in production of iron oxide cinder predominantly FeaOl. In suspension roasting of sulfide fines it is possible to generate much more heat than is necessary to maintain the reaction exothermically self-sustaining.

At temperatures of about 850 F. ferrous sulfate decomposes endothermically to form iron oxide predominantly FezO: and about equal amounts of SO: and S03. 'I'his reaction proceeds very slowly at this temperature. It has been found that under conditions existing ln suspension roasting, with temperatures of the order oi 1850 l". prevailing, decomposition of iron sulfate proceeds rapidly and that S0: is dissociated to SO:

and oxygen. In customary sulfide fines suspension roasting, it is necessary to limitthe amount of nnes fed into the combustion chamber so that the gas temperatures will not exceed the permissible upper limit, e. g. 1900" F. In the process of .the present invention, all of the heat in the burner-over and above that lost with the cinder and by radiation, and that required to maintain the roasting operating temperatureis uuiimed to effect decomposition of iron sulfate. Developments upon which the invention are based show that when iron sulfate fines are introduced along with sulfide fines into a commercial suspension roaster of the type shown in the drawings and having a height suchithat the lines had a drop of about 25 feet, the iron sulfate is decomposed with the production of an iron oxide cinder containing as little as 0.5% sulfur. While normal endothermic decomposition of iron sulfate at temperatures of about 850 F. results in production of a cinder predominantly FezOx, o'n account of the much higher temperatures present in the suspension roasting operation the iron oxide cinder produced by decomposition of the iron sulfate is predominantly FeOu, i. e., the same form as the iron oxide cinder resulting from suspension roasting of the pyrites fines.

Degree of subdivision of the pyrites may vary over a relatively wide range, preferably not coarser than about 60 mesh. Fineness of the iron sulfate is likewise variable although indica;

tions are that better desulfurization of ironfsulfate is obtained where the particle size is'less than mesh andpreferably of the order of" -200 mesh. In the present speciiication and claims the term finely divided" isused to define a degree of subdivision suflicient to permit commercially satisfactory desulfurization of either iron suliide or iron sulfate. It will be appreciated that height of the burner and the mode of injection of the fines, whether downwardly from the top or upwardly from the bottom or otherwise, iniiuence the time inter'val during which the fines are maintained in suspension. When using some types of burners the iron sulfide and sulfate should be more finely divided than in the case of others so as to permit lapse of a suilicient time interval in which to complete roasting of the fines while in suspension. The sulfide and sulfatevmay be charged to the burner by means of separate injectors, although usually better results may be obtained when the fines are introduced in admixture as described.

Reaction conditions in shaft furnace l0 are controlled by regulation of the relative quantities of pyrites iines, iron sulfate fines, and oxygen introduced. The amounts of oxygen and pyrites and sulfate fines charged are regulated so that exothermic combustion of the iron sulfide is effected in such a way as to maintainvin the combustion zone temperatures about or above dissociation temperature of sulfur trioxide but vpreferably not above temperatureswhich would cause the iron oxide cinder at the poit of discharge from the roaster to fuse and set up as a solid cake. By maintaining these conditions, the iron sulfide fines are roasted to produce iron oxide (Fes04) cinder and SO2 gas; the iron sulfate is decomposed to form iron oxide (EeaOl) cinder,l SO: land S0: gases.. The S03 is disso- -clated to SO: and oxygen; and the sulfur and iron values of the iron sulfide and of the iron sulfate are recovered as a concentrated SO: gas and as relatively free-flowing iron oxide cinder.

The quantities and relative proportions of iron sulfide and sulfate charged to the burner are iniluenced by several factors such as specific compositions of the sulilde and sulfate, the size and presence in the combustion zone of as much heat ated sulfuric acid plant.

insulatioh of the burner, initial temperature of the combustion supporting air, and .SO2 concen;

tration desired in the burner exit gas.

With respect to the amount of sulfide fines introduced, it will be recalle,y that in normal suleildesu'spension roasting, us g pyrites alone, feed of pyrites is limited for a. given burner so as to avoid excessive gas temperatures in the combustion chamber. In the present process, no such limitation is encountered. To the contrary, the

as possible is desirable because the greater the amount of heat available the greater may be the amount of iron sulfate decomposed with resultant increased SO2 gas concentration and increased overall capacity of the burner and of the associ- Thus,- giving consideration to other factors involved such as size of the furnace,linitial temperature of the oxidizing gas, and amount of iron sulfate available for use, the quantity of iron sulfide fed into the burner in the present process is determined according to specific operating conditions existing so as to maintain the reaction as a whole exothermic and to maintain in the combustion zone temperatures, as nearly as practicable, of about 1850 F.

'I'he total air used in the burner in the present process may be considered asV the sum of two increments: (l) the amount of air necessary 'to loxidize the pyrites to FeaO'i cinder and gas con-v taining SOz, Oz, and Na; and (2) an additional amount of air to preferably insure presence in the burner exit gas of the same SO2-oxygen ratio as is present in an SO2 gas from pyrites alone. For example, assume the first increment consists of air required to roast the pyrites present and produce a 12% SO2 gas. Thus, combustion of pyrites to form a. 12% S02 gas results in formation of iron oxide cinder chiefly Fe304 and a gas containing (dry basis) about 12.0% SO2, 5.7% Oz. and 82.3% N2, which represents an SO2 to oxygen ratio of 1 to 0.47. During roasting of pyrites, decomposition of FeSO4.HzO simultaneously takes place producing iron oxide chiefly FeaOr and a gas which after condensation of the water vapor would contain about 75% SO2 and 25% O2. I In this situation, assuming use of .air preheated to 900 F., upon mixing the pyrites gas and the iron sulfate gas there would be formed a combined burner exit gas containing (dry basis) 15.9% SO2, 6.9% O2, and 77.2% N2. Since the SO2-oxygen ratio of the burner exit gas in the instance mentioned is 1:0.43 a second increment of air should be introduced into the combustion chamber in amountenough to dilute mecumbustion gases so that the combined burner exit gas hasan SO2-oxygen ratio of about 1:0.47.

,The amount of iron sulfate decomposed in a given furnace depends upon the total amount of heat available, and the latter in turn depends upon the amount of iron sulfide burned and upon the temperature of the air as fed into the burner. One of the surprising features of the invention is the unexpected eife'ct of use of preheated air on the capacity or sulfur output of a given furnace. SeveraLprior art suggestions have been made relative to use of preheated air in suspension roasting. Such suggestions are for the most part based on assumption that use of preheated air is beneficial with respect to furnace capacity. However, commercial practice of suspension roasting has shown to the contrary. In Example 1 TableI, using pyrites alone,v a given burner and is normally about 100 F.) the maximum sulfur output of the burner is 24.6 tons per day (Item 7). Practical experience has shown that in the case of most ores one of the major diiliculties in suspension roasting is that too much heat is developed. If more heat units are introduced into the burner in the form of preheated air, since there is already enough required heat present in the combustion zone, it will be seen the only way to avoid excessive gas temperatures is to cut down the sulfide ore input sufficiently togerierate a lesser amount of heat equivalent to heat units introduced with the preheated air. For linstance, if in Example 1 air were preheated to about 900 F., in order to avoid excessive gas temperatures in the-burner it would be necessary to cut down the sulfur input about 65%, that is, from the 25 tons (Item 5) to about 8.6 tons. Hence, in usual suspension roasting of pyrites,

) preheated air is detrimental and a practical oppreferred embodiment of the invention and of the results obtained. In the present process, by reason of use of iron sulfate in conjunction With the sulfide fines roasting, vit is possible to make use of all of the heat which can be generated by burning sulfide fines. Hence, heat radiation from the burner shell need not be relied upon, as in the prior practice, to provide for dissipation of the excess heat.

as well as feasible.

In the present preferred embodiment, the amount of air introduced into the burner is such as to produce at the burner exit a gas containing (dry basis) 15.5% S02, 7.3% O2, and 77.2% N2. Such gas passes upwardly through supplemental combustion and dust settling chamber 35, Fig. 1 of the drawings, enters the top of exchanger 40 at about 1750 F., leaves thru pipe 4'6 at about 1275 F., andis discharged from the top of exchanger 50 at about 700-750 F. At this ternperature the SO2 is relatively non-corrosive and hence a cyclone dust separator 65 made of ordinary sheet steel may be employed. This 15.5% Soz'gas (corresponding with the 10% SO2 gas Accordingly, in the present processit is desirable to insulate the burner shelll Item 10, Example 1) is fed to the head of the wet purification system 61, Item 11, Example 6. Item 12 shows that on account of the largely increased SO2 concentration a 35.5% reduction in size of the purification system is made possible. The 15.5%.SO2 exit gas of wet purification contains insufficient oxygen to facilitate commercial oxidation of SO2 in the converter system. Introduction of sufficient air through inlet |33 to provide in the gas stream a ratio of one volume of SO2 to 1.1 volumes of free O2 reduces the SO2 concentration to about 10.6. summarizing the major results made possible by the invention, using the same burner as in the standard operation, the sulfur output of the burner may be increased from 24.6 tons (Item 7, Example 1) to about 38.1

tons (Item 7, Example 6), the relative capacity.

or sulfur output of the burner is increased from about 100 to about 155% (Item 8), the purificacation system may be made 35% smaller, and u Vthe drying, converter and absorption units may be made 20% smaller.

In Example 6 thel weight of FeSO4.HzO and iron pyrites charged into the burner was about 1.1 to 1.. It will be appreciated the amount of iron sulfate which may be put through the furnace may vary within rather wide limits depending principally upon the composition of the metal suldefor example whether iron sulde or zinc sulde, upon the degree of preheat of the air fed to the burner, eiectiveness of insulation employed, and upon the amount oi FESO4.H2O available ior any speciilc burner operation.

'Ihere may be instances in which a relatively limited quantity of iron sulfate is available and in such case the degree of preheating of the air may be reduced or air preheating dispensed with entirely.

Fig. 2 of the drawings illustrates a plant layout for use in a case where only a limited amount of iron sulfate is available.- In this situation, heat is taken out of the system by means of a waste heat boiler 94 instead of being put back into the. burner in the form of air preheat as in the system illustrated in Fig. 1. In the present process, the more or less one molecule of water included in the iron sulfate is vaporized in the burner. In instances where a waste heat boiler is employed it is advisable to dry the combustion supporting air to avoid condensation of HzSO4 in the boiler. In practice of the process as carried out in ap/ paratus of Fig. 2, air is drawn into thesystem yby a blower 80, passedthrough a standard drying-tower 8l and iiowed thence through conduit 82 to burner air inlet 30. In fur-nace 12, roasting of suliide and decomposition of iron sulfate A is generally the same as in burnerjll of Fig. 1.

Example 5 of Table I illustrates an embodi.

ment of the invention which may be carried out in apparatus of Fig. 2 and where air for combustion is not preheated. However, in Example 5 since no heat units are brought into the burner inthe form of -preheated air there is less heat available for decomposition of iron sulfate and accordingly the tonnage of iron sulfate is reduced from 60 of Example 6 to about 29 (Item 4). Example 5, about 0.5 1b. of FeSO4.H2O is fed into the furnace per pound of pyrites lines (Item 6). 'I'he gas leaving the burner` through outlet 84 (Fig. 2) contains about 13.7% SO2 (ItemA 10), 6.5% O2, and r19.8% N2, and is at temperature of about 1850 F. Such gas ows upwardly through supplemental combustion and vdust collecting chamber 81, and enters the top of waste heat boiler 8l at about 1750" F. The boiler is controlled so that the gas leaving through pipe I is' at temperatures of about G50-700 F.

The basis for the improvements afforded by the invention is utilization of excess heat in the burner to decompose iron sulfate. vThe more excess heat present the greater may be the proportionate amount of iron sulfate contained in the iron sulde-iron sulfate furnace charge, with the result that the greater `is the proportionate amount of H2504 which may be produced from iron sulfate. I'n Examples 1 to 6, Table I the iron sulfide was pyrites comprising approximately 46% iron and 50% sulfur. Pyrrhotite, which may be considered as comprising 58% iron and 35% sulfur, is another sulfide ore well adapted for suspension roasting. Ii.' the standard burner of Example 1 were changed over to operate on pyrrhotite alone (not using ironsulfate) instead of pyrites alone, on account of the greater iron and lesser sulfur content of pyrrhotite as com-` pared with pyrites. the. relative sulfur output of the burner would be substantially decreased, an'd the plant H2804 production would be cut correspondingly. Hence, an operator is placed at a marked disadvantage if, for any reason. he switches from use of pyrites alone to pyrrhotite alone. However, it has been found, as another surprising feature of the invention, that 'when roasting iron sulfate in conjunction with pyrrhotite the advantages are still greater than when roasting iron sulfate in conjunction with pyrites. Thus, in the present invention use of pyrrhotite as the sultlde is not a disadvantage (as is the case when substituting pyrrhotite alone for pyrites alone) but is actually a material aclvantage. 'I'his advantage is based on the fact that when, in the present process, pyrrhotite is used a higher proportionate increase in gas concentration is obtained and the proportionate part of the ultimate H2804 product made from iron sulfate is substantially greater than when using iron sulfate and pyrites.

The reasonwhy an operator is placed ata disadvantage with respect to H1804 tonnage production when switching thev given standard burner Aover from burning pyrites alone to burning pyrrhotite alone will be understood from the following.

Example 1 of Table I and Example 'I of Table II respectively show a comparison of operation of an identical burner with pyrites alone and pyrrhotite alone. Burning pyrrhotite alone in conventional hearth roasting results in production of a 'L5-8.0% SO: gas corresponding with the 9.5-10%' gas produced when roasting pyrites alone by conventional hearth roasting. Thus ir( Example 7, it is assumed the representative' suspension roaster, burning pyrrhotite alone. is operated so as to produce an 8% SO: gas (Item 10). This 8% SO: pyrrhotite gas (as compared with the 10% S01 pyrites gas Item 10, Example 1) requiring larger purication, contact and absorption systems is the rst disadvantage encountered by an operator changing over to pyrrhotite.

Pound per pound pyrrhotite contains a greater amount of iron and lesser amount of sulfur than pyrites. Accordingly, a greater amount of oxygen is used to oxidize the iron of pyrrhotite than in the case of pyrites, and there is a lesser amount of sulfur available for combination with oxygen to form SO2 than in the case of pyrites. When operating with pyrrhotite alone, just as inthe case of operating with pyrites alone, the exit burner gas should be maintained at, e. g.1850 F., and this temperature is controlled by the amount of pyrrhotite charged. In burning pyrrhotite to an 8% SO: gas so many heat units go into the gaseous products, because of additionalheat units generated in oxidizing the greater 'ironcontent and because of the smaller air volume required per pound of pyrrhotite, it would be necessary when switching the standard burner over from pyrites to pyrrhotite to cut the 50 ton ore charge (Item 3, Example 1) to 41.8 tons (Item 3, Example '7). That is, when burning pyrites alone 50 tons of pyrites may be charged, but when burning pyrrhotite alone in the same burner in order to mainvtain in the burner exit gas the conventional 'amount of free oxygen and to maintain the say contains about 30% lesssulfur than pyrites. The 15 net result is that when burning pyrites alone the sulfur output of the burner is 24.6 tons and the burner capacity or sulfur output is 100% (Items 7 and 3, Example 1), whereas when burning pyrrhotite alone in the same burner the sulfur loutput is reduced to 14.2 tons and the burner capacity or sulfur output is reduced to 58% (Items 7 and 8, Example 7). Accordingly, when an operator changes over from pyrites alone to pyrrhotite alone, the second disadvantage is that the burner sulfur output (as sulfur or as SO2) is reduced from 100% to 58% correspondingly cutting the plant H2SO4 production from 100% to 58%. The disadvantages of changing a-burner over from pyrites to pyrrhotite are plainly evident, and the extent of these disadvantages may be more fully recognized when it is appreciated that in some localities pyrrhotite is theonly sulfide ore available.

In Table II the burners of Examples 7-10 are identical with the burners of Examples 1, 2, 5 and 6 of Table I. However, in Table II since` the initial burner gas is 8% SO2, the purification systern of Example '7 is designed to handle an ,8% pyrrhotite gas. Such gas contains about 8% O2, and more oxygen should be introduced into the gas stream just ahead of the drying system to provide the 1:1.1 SO2 to oxygenl ratio. After introduction of air to provide the requisite oxygen, the SO2 concentration of the gas entering the drying system would be reduced to about 7.5% (Item 15, Example '7).

Example 7 thus typifies what may be considered a standard operation using the given standard burner and operating on a maximum capacity for such burner on pyrrhotite ore. When roasting pyrrhotite alone, it would be to the operator's advantage to produce stronger SO2 gas s0 as to' make possible use of a smaller purification system as in the case of pyrites. Assume it is desired to produce `a SO2 pyrrhotite gas (the maximum SO2 concentration obtainable by suspension roasting of pyrrhotite alone) to`permit use of a smaller purification system. Example 8 of Table II illustrates such operating conditions and shows the burner capacity or sulfur output is reduced from 58% to 37% (Item 8).

If the operator desires to run the burner on pyrrhotite to produce a 10% SO2 burner exit gas and obtain the benefit of the 20% size reduction of the purification system and still maintain the same H2804 plant production as in Example 7 he has two alternatives available: first, to use the given standard burner plus a second burner about 55% as large as the given standard; or second, to

use a single new burner about twice the size of the given standard. In other words, the alternatives available when burning pyrrhotite are the same as those possible when burning pyrites as illustrated in Examples 3 and 4 of Table I. The 'objections to the alternatives in the case of roasting pyrrhotite alone are the same as those when roasting pyrites alone.

While combustion of 1 lb. of pyrites generates a slightly greater number of B. t. u.s than does the combustion of 1 lb. of pyrrhotite, oxidation of 1 lb. of Dyrrhotite to Fea04 and SO2 requires a smaller weight or volume of air` than does oxidation of l lb. of pyrites to Fea04 and SO2.l The result of this is that in the roasting of pyrrhotite because of the additional B. t. u.s generated by oxidizing the substantially greater iron content and because of the smaller air volume required per pound of ore, there is produced in the roasting of pyrrhotite a greater number of B. t. u.s per pound o: gaseous reaction products than when roasting pyrites. Accordingly, although while roasting pyrrhotite alone possesses substantial overall plant capacity disadvantages as compared with pyrites roasted alone, in the roasting of pyrrhotite alone there is more excess heat available for raising the temperature of the reaction zone than in the case of roasting pyrites alone. Accordingly, when roasting pyrrhotite, there is more heat available fordecomposing iron sulfatewith the result that on applying the principles of this invention to roasting of pyrrhotite, a greater proportion of iron suifate may be decomposed vand a greater proportionate amount of ultimate H2SO4 product may be made from iron sulfate than when using pyrites as the suliide ore. While in prior practice it was a disadvantage to roast pyrrhotite alone, the present invention substantially eliminates the gas concentration disadvantage and creates an advantage with respect to the amount of ultimate H2804 product made from iron sulfate.

In the present modification using pyrrhotite as the sulfide ore, maximum economies may be obtained where the air is preheated. Example 10 illustrates an embodiment of the invention using pyrrhotite, air preheated to about 900 F. and the plant layout of Fig. 1 of the drawings.

The burner exit gas, containing (dry basis) 4about 15% SO, (Item 10, Example 10), 7.3% O2,

and '77.7% N., and heated to temperatures of about 1850 F., passes through chamber 35, enters the top of heat exchanger 40 at about 1750 F., is introduced through pipe 46 into the bottom of exchanger 50 at temperature of about' 1275 F., and is discharged .from the top of exchanger 50 through pipe 63 into dust collector 65 at temperature of about 70D-750 F.

Example 9 illustrates an embodiment of the invention using pyrrhotite and the plant llayout of Fig. 2 of the drawings. Atmospheric air is drawn into the system by blower 80, dried in tower 8 I, and introduced into furnace air inlet 30 at temperature of about 100 F.

The burner exit gas containing (dry basis) about 13.2% S02 (Item 10, Example 9), 6.5% 0 80.3% N and heated to temperatures of about `1850a F., passes through chamber 81 and at about 1750" F. enters the top of Waste heat boiler 94 controlled so that the gas stream is discharged into conduit |04 at temperature of about 70D-750 F. and fed into cyclone dust separator 13.

In Examples 5 and 9, using a well insulated` burner and unpreheated air, iron sulfate and sulfide ilnes were used in proportions of about half pound of vsulfate to one pound of cre. Where smaller amounts, say half as much iron sulfate is available, the process may be employed to'similar advantage. However, as the relative proportion of .sulfide is increased, the amount of heat generated is correspondingly increased, and in this situationy it is only necessary to less effectively insulate the burner sufiiciently to dissipate by '.since the contained carbon would consume so much oxygen in the roasting operation and form so much Co. that the vSO2 concentration of the resultant roaster gas would be low and comparable with that obtained from pyrrhotite. While the V8O. concentration of suspension burner exit gases when using pyrites or pyrrhotite alone is normally higher lthan when roasting pyrites or pyrrhotite alonel in a hearth roaster the same objections relative to the use of coal brasses in hearth roasters applies to suspension roasters, namely, dilution ot the SO, gas by CO2. Consequently, although coal brasses run very high in sulfur this material is of little value as compared with straight pyrites and is accordingly available on the market at much lower prices.

When carrying out the present proc using coal brasses as a substitute for straight pyrites or pyrrhotite, roasting of the pyrites content of the brasses maintains the exothermic nature of the reaction and also generates excess heat which is utilized to decompose iron sulfate. Although combustion of thecarbon content oi the brasses w CO. generates substantial amounts of heat, such heat units are not needed to maintain the exothermic nature of the reaction but are available to decompose still further quantities of iron sulfate. Decomposition of iron sulfate produces a very strong SO. gas which after condensation of water vapor would contain about 75% SO: andy 25% oxygen. Such SO. gas before introduction into the converters, must in any event be substantially diluted either by air introduced into the burner or fed in to the gas stream just ahead of theV drying unit or both. Consequently, in present modification of the invention using coal brasses the diluting effects of the CO: formed by combustion of the carbon content of the brasses are unobjectionablel For example, FeSO4.HzO and coal brasses (assumed to contain about carbon by weight) may be introduced into the combustion chamber in proportion of about 0.78 lb. of m8041120 to 1 lb. of coal brasses. When using air initially at atmospheric temperature and operating to maintain the burner exit gas temperature at about l850 F., the resulting burner gas may contain (dry basis) 12.7% SO 6.0% Oz, '18.1% Nn and 3.2% CO2. It is thus possible, in spite of CO. dilution, to produce a concentrated burner gas. In this instance, the H504 production from the brasses amounts to about 74% of the total, and the H1804 production from FeSOaHzO is about 26% of the total.

Still greater consumptionoi iron sulfate may be had where the ai'r introduced into the burner is preheated. Assuming air preheat of about 900 F., FeSOaeHzO and coal brasses may be fed into the combustion chamber in proportions of about 1.37 pounds of FeSO4JHzO to 1 lb. of brasses. The burner exit gas may contain (dry basis) 14.4% 80s, 6.8% O. 75.8% N., and 3.0% CO1. The H2504 production from brasses may comprise about 62% of the total and the H2804 production from FeSO4H2O about 38% of the total.

The above description applies to .a coal brass concentrate which is produced in relatively small quantities since the cost of equipment and operation adords little economic justification for application of concentration process. However, in some bituminous elds, where soft coal as mined contains from 2% to 4% sulfur, the coal is sub- Jected to a washing operation resulting in production of clean coal and large quantities of a ref' use coal which may be as high as -22% sulfur. It is this refuse material that is subjected to con- ,Il centration methods to produce the previously water.

referred to "brasses concentrate. The refuse is unsaleable for coal on account of the high sulfur content, and is not usable for manufacture of HSOi by the customary roasting methods because of the high carbon content. However, such refuse may be employed in the present process since the heat developed by combustion of the higher,r carbon content may be used to decompose further amounts oi iron sulfate, the strong SO: produced from iron sulfate oiisetting CO. dilution.

' The invention thus aliords two very distinct additionaly advantages: iirst, in that it makes possible economlc utilization of a very low-priced raw material, i. e., coal brasses or coal refuse: and further, that when employing such materials the Y proportionate part of the total H2804 prpduction from iron sulfate is greater than when using iron sulfate in conjunction with either pyrites or pyrrhotite ore.l Another outstanding advantage of h the process of the invention is that when using, in conjunction with the sulfate, either pyrites,

pyrrhotite or pyrites containing coal it is possible to produce an SO2 gas as strong as can be made by burning brimstone, the highest grade of sulfur raw material.'

The principles of the invention as described in connection with use of iron sulilde and iron sulr fate apply equally to use of zinc sulde ores and zinc sulfate. It is preferred to use the sulilde and sulfate of the same metal so that the burner cinder will consist of the oxide of asingle metal. However, where the situation is such that cinder comprising the oxide of morethan one metal is unobjectionable, the sulde'in the present process may be the sulfide of one metal, and the sulfate the sulfate of another metal. 'I'he higher oxide 1 o! the sulfate, such as ferric sulfate may also be used in place of the lower oxide such as ferrous sulfate. Elemental sulfur may be substituted for sulilde ore, although for practical purposes there would be little or no reason in using a relatively pure, high-priced raw material in place of the cheaper sulfide.

The iron sulfate mentioned in the foregoing examples contains approximately one molecule of water. For practical purposes the monohydrate is preferred because the water content is relatively low and vaporization of such amount of 4water in the burner does not consume objectionable amounts of heat. Further, the one water salt is readily obtainable by known crystallizing -processes. The anhydrous sulfate may oi' course be used to even greater advantage than the monohydrate. In the appended claims the expression relatively dehydrated sulfate is intended to include a salt containing about one or less molecules of The water contentof the iron or other sulfate used in the lpresent process is not critical but is more an operating feasibility. 'I'he principles of the invention likewise apply when the salt contains more than one water, e. g., copperas. However, in this situation much heat is needed to vaporize the waters with the result that the quantity of FeSO4 which may be decomposed would be substantially decreased. If the sulfate is initially available in the seven water form it is more advisable to preliminarily dehydrate down to about one water in a suitable dryer.

While theroasting operation per se has been A I 5 Vltexn Example 1 Example 2 Example 3 Example 4 Example 5 Example 0 l 1. Tlnpt. 011111111011101 au 100 F 100 F 100 1r 100 r 100 r 000 F.

e A l 2 Type o1 burner..A Given standard- Given stand- Given stand- A single newl *Given etand- Given standj ard. ard plus a b u r n e r ard well inardwellinsu- 10 s e c o n d about 2.5 minted. lated. burner `70% times the as large as site olgiven irenetandstandard. s omhargenetwmperday. so o 29.2- 00009101? 01 5o 0 um 15,0,

vers)- 15 4 FeSOIhO charge, net tone 0 0 0 o 29 o w 5 per ey. 5 Sulfur to burner, net tons 25.0 14.0 25.0 (total of 25.0 l 33.0 33 9, per day. botn burners 0 Pounds of FeSO4.H1O per 0 0 0 n 5 1.1.

pound of sulfide ore. 2- 7 Sulfur ex burner, net tone 24.6. 14R 24.6 (total of 24.0 32.3--. 38.1.

perday. bot)h bumers 8 Relative 'burner capacity 10002.` 58% 1007 (totaloi 100% 131% 155 (sulfur output). 1131311 burners 9 Temp. of S01 gas at burner 1,850 F 1,850 F 1,850 F 1,050 Il 1,850' l' 1,850 F. 2s 1o- 11019611081111011011 11111111101 10% 12/n 12% 12en 13.7% 10,0%.

exi l, l1.- S01 concentration in wet 10%` 12% 12o/1 12% 13.7% 15.5%.

purification plnnt (coolers, i scmbbers, ete). l 12-..-- Percent reduction in size o! None (given 16.7%.. 16170/1- 10.7% 27.0%l 35.5%.

puriiication system. eine for 10%. 30

. S05 pylitiw ges 13.- Percent ofadditionsl H1304 None m95 20% 20!!- 0705- 55%.

producible in given pnri- Y l llcetion plauto increased S01 conoentrat on. 14----. Air introduced at head of Yes Yes Yes Yee Y Yee.

drying unit. 15----- B01 concentration in dry- 8.5% 8.5% 0.5% 3.5% 9.70%. 10.0%.

g1g, cenitserter and abeorp- 0X1 4 16.---- Percent reduction in size of None (given None-.. None None 12.0% l 00. 0. drying, converter and ebeine for 8.5%

sorption unita. B 0)1 pyrtcs T43-F1 17--... Percent of additional H1304 None None..- None--- None 140!I 25%. m

inducible in given dryconverter and absorption units by in S01 concentration. 18--..-- H1804 production trom ore.- 100% 100% im 100% 88.4% 70.57 10----. -H1SO4 production from 0 o 0 0 16.09?- m.

f FBSOnHsO. i J,

Tenu H Suljldeore used. Pyrrhotite (FevSa, 58% Fe, 35% S) non Example '1 Examples nmnpne empl@ 1o 1.---.- Temp. of air at burner air inlet--. 100 ILV.- 100 F 100 F 900 F. 55

--. 'Iypeoi burner Given standard iden- Given staud- Given stend- Given standtical with "atan d ard. ard woll inard weil inoi Table I). enlisted. saluted. Ore charge, net tons per dayf 4l 8 27.0 67.0 07.0. FeSO4.H1O charge, net tons per day n 0 41.5 73.7. Sulfur to burner. net tons per dey` 14.0.--- 0.1L..- 31 2 37.3. Pounds of FeS01.H1O per pound of sulfide n n o e 1.1. 60 Sulfur ex burner, net tonsper day 14 2 9.2 30 3 38.2. Relative burner capacity (sulfur output) 58% 37% 123% 147%. Temp. of S01 gas at burner exit 1,850 F 1,850" F 1850 F 1,8m" F. S01 concentration at burner exit-- 807 10% 1 13g? 15.07 S01 crentratiln in wet purification plant (coolers, S 1001 13 o, 15.0 a.

Seru rs, e v Perent reduction in size oi puriication system None (given size for 8% 20.0% 39.4% 40.7%. 65

S01 pyrrhotite gas). Percent of additional H1804 produclble in given puri- None-- 25% 65% 875%- ticetion plant by increased S01 coneentration. Air introduced at head of drying unit Yes Yes Yes es. Stg; conclietrtraton in drying, converter and absorp- 7.5% 7.5% 9.5% 10.4%

0111111 Percent reduction .in size of drying, converter and None (given size for None.---';..-- 2i. a 23.0%.

absorption units. 7.5?. B01 pyrrhotite 70 L* sas Percent of additional H1S04'producible in given dry- None.-- None 2194- 39%- inlz, converter and absorption units by increased so, concentration. 18-.--- H1804 production from ore 100% 100%. 75% 02. 19-..-- H1804 production from FeSO1.H1O 0 0 24. 37.4

v m .u

`sumde ore used.- zron mires (Fesz, 46% 1re, 50,00 s) I claim:

metal sulfatefiines and oxidizing gas intova combustina zone heated to temperatures above the ignition point of the suliide fines, forming in the `combustion zone a dispersion of the suliide and sulfate fines in the oxidizing gas, roasting the sulde nes while in gaseous suspension, regulating the amount of sulfate kintroduced so as to avoid reducing combustion zone temperature below that at which sulfide fines introduced into the combustion zone ignite and oxidize in the presenceot oxygen, causing the sulde and sulfate nnes to pass through the combustion zone so as to maintain such ilnes in suspension for a suilicient time interval to roast the suliide fines and to decompose the sulfate fines to form from the sulde and sulfate ilnes sulfur dioxide gas and substantially desulfurized metal oxide cinder, withdrawing sulfur dixode from the combustion zone, and discharging cinder therefrom.

2. 'Ihe method for roasting metal suliide fines which comprises introducing metal sulfide lines, metal sulfate iines and oxidizing gas into a combustion zone heated to temperatures above the ignition point of the sulilde fines,` forming in the combustion zone a dispersion of the sulde and sulfate ilnes in the oxidizing gas, roasting the sulde fines while in gaseous suspension, regulating the amount of sulfate introduced so as to avoid reducing combustion zone temperature below that effecting substantial dissociation of sulfur trioxide, causing the sulfide and sulfate fines to pass through the combustion zone so as to maintain such ilnes in suspension for a sufficient time interval to roast the sulde fines and to decompose the sulfate nes to form from the sulfide and sulfate ilnes sulfur dioxide gas and substantially desulfurized metal oxide cinder, withdrawing sulfur dioxide from the combustion zone, and discharging cinder therefrom.

3. The methodl according to claim 1 in which the amount of sulfate introduced is regulated so as to prevent combustion zone temperature above g that causing metal oxide cinder at the point of discharge from the combustion zone to form solid cake.

4. The method according to claim 1 in which the oxidizing gas is preheated.

5. The method according to claim 1 in which the fines are sulde and sulfate of iron.

6. The method for roasting metal sulfide iines which comprises introducing oxidizing gas, metal sulde fines and relatively dehydrated metal sulfate fines in proportion of about one to not less than about 0.5 by weight into a combustion zone heated to temperatures above the ignition point of the suliide iines, forming in the combustion zone a dispersion of the sulde and sulfate lines in the oxidizing gas, roasting the sulilde nes while in gaseous suspension, causing the sulfide and sulfate nes to pass through the combustion zone so as to maintain such ilnes in suspension for a suilicient time interval to roast the suliide fines and to decompose the sulfate lines to form from the suliide and sulfate nnes'sulfur dioxide gas and substantially desulfurized metal oxide cinder, withdrawing'sultur dioxide from the oombustion zone, and discharging cinder therefrom.

'1. 'rnc vmethod according tc claim s m which the amount or summe introduced 1s regulated sc as to prevent combustion zone temperature above that causing metal oxide cinder at the point of discharge from the combustion zone to form solid cake.

8. 'I'he method according to claim 6 in which the oxidizing gas is preheated at least several hundred degrees F., and the sulilde lines and suifate nes 'are in proportion of about one to not less than about one by weight.

9. The method for roasting nely divided combustible solid sulfur bearing material which comprises introducing said material, metal sulfate fines and oxidizing gas into a combustion zone heated to temperatures above the ignition point of the sulfur bearing material, forming in the combustion zone a dispersion of the sulfur bearing material and sulfate iines in the oxidizing gas, roasting the said material while in gaseous suspension, regulating the amount of sulfate introduced so as to avoid reducing combustion zone temperature below that at which the sulfur bearing material introduced into the combustion zone ignites and oxidizes in the presence of oxygen,

` causing the sulfur bearing material and Vsulfate 'the fines are sulfide andy sulfate of the same metal.

11. The method according tc claim 1 in which the sulfate ilnes are relatively dehydrated.

12. The method according to claim 2 in which the amount of sulfate introduced is regulated so as to prevent combustion zone temperature above that causing metal oxide cinder at the point of discharge from the combustion zone to form solid cake.

13. The method according to claim 2 in which the ilnes are sulfide and sulfate of iron.

14. The method according to claim 2 in which the metal sulfate fines are relatively dehydrated, and metal suliide fines and relatively dehydrated metal sulfate fines are introduced into the combustion zone in proportion of about one to not less than 0.5 by weight.

l5. The method according to claim 6 in which the nes are sulde and sulfate of the same metal. I o

16. The method according to claim 9 in which the finely divided combustible solid suli'ur 'bearx ing material is coal brasses.

BERNARD M. CARTER. 

